1. Field of the Invention
The present invention relates to a radiation dose rate measurement system to be installed in nuclear reactor facilities spent fuel reprocessing facilities or the like, and around such facilities.
2. Description of the Related Art
In order to measure dose rates over a wide range from a natural radiation level to a high radiation level during a fault of a nuclear reactor or the like at both the inside/outside of nuclear reactor facilities, a plurality of dose rate measurement systems according to radiation levels need to be used in view of sensitivity and accuracy. Accordingly, it is required to enable measurement which responds to wide range dose rates by a single dose rate measurement system.
For example, in a gamma ray irradiation dose measurement system of a wide range described in JPS61-104282A, irradiation dose rates over a wide range can be measured by a single measurement system by providing a lead shield with special structure to a sodium iodide activated by thallium (NaI(Tl)) scintillation detector and by automatically selecting measurement by a discrimination bias modulation (DBM) measurement method (pulse height discrimination bias modulation method) by a pulse signal proportional to an irradiation dose of incident gamma ray from the detector at a low dose rate or measurement by a current measurement method at a high dose, depending on the level of an irradiation dose rate at a measurement place.
More specifically, the gamma ray irradiation dose measurement system includes: the NaI(Tl) scintillation detector which absorbs incident radiation energy, sends a discrete current pulse having electric charge proportional to the absorbed energy, and converts a direct-current (DC) current to a direct-current (DC) voltage to be sent; and a measurement unit in which the current pulse and the DC voltage are applied to be converted to dose rates to be sent. The measurement unit includes: a preamplifier which converts the current pulse to an analog voltage pulse to be sent; a main amplifier which amplifies the analog voltage pulse and removes high frequency noise; a DBM circuit in which the amplified analog voltage pulse is weighed by a dose rate and sends pulses of cyclic frequency proportional to the dose rate; a voltage/frequency (V/F) converter in which the DC voltage is applied to be converted to pulses of cyclic frequency proportional to the voltage to be sent; a gate circuit in which these pulses are applied and switched to be sent; and a calculation section in which the output pulses of the gate circuit are applied to calculate a low range dose rate based on the pulse of the bias modulation pulse height discrimination circuit or a high dose rate based on the pulse of the V/F converter to be sent, and to switch the gate circuit by a set dose rate. Energy characteristics as errors generated depending on gamma ray energy are different between a low range dose rate and a high range dose rate, and switching is made by a fixed dose rate with respect that an optimum switching point between the low range dose rate and the high range dose rate is different depending on the gamma ray energy; and accordingly, a large level difference is generated at a switching point and thus the shield with special structure is provided on the NaI(Tl) scintillation detector in order to reduce the large level difference as much as possible.
However, in the gamma ray irradiation dose measurement system described in JPS61-104282A, both the error caused by the energy characteristics of the high dose rate range and the level difference generated at the switching point can be reduced to some extent by providing the shield with special structure on the NaI(Tl) scintillation detector; however, a problem exists in that the original energy characteristics of the low range dose rate deteriorate due to widening range.